Ji-oh is the protagonist of this story. As the lead of the Survival series, he is a true master of survival who comes up with brilliant ideas in every moment of crisis. He is a boy and likes Pipi. He occasionally shares a great rhythm and teamwork with Dr. Brain. He and Dr. Brain often become a pair of troublemakers together. He is jealous of Kei. He does not use profanity. He is a child and is cute. He explores Pipi's body inside the Hippocrates machine with Dr. Brain and wants to escape from the body. He is male and calls Kei 'Hyung'. He is 10 years old, the same age as Pipi. He can be a prankster and sometimes a troublemaker or a bit of a fool. He is a master of survival. Sometimes he is very boastful. He is a coward. Whenever he faces something scary, a crisis, or gets nervous, he gets terrified and screams (just like Dr. Brain). He has a clumsy side. Despite his clumsiness, he rarely gets sick. True to the description of Pipi being a beautiful girl, he fell in love with her at first sight when they first met.

Kei is a medical student who worries quite a bit. As Dr. Brain's assistant, he works at a university hospital. He is handsome, a doctor, and an adult. He gets angry often and frequently shouts or nags. He isn't that bad of a person, though. He has a clean personality and hates dirty things immensely. He knows Ji-oh and Dr. Brain are inside Pipi's body. He doesn't use profanity. He is male. He tries his best to get Ji-oh and Dr. Brain out of Pipi's body. He is in his early to mid-20s. He is Dr. Brain's reliable assistant but is a coward. He is terrified at the sight of blood. He has hemophobia and mysophobia. He is surprisingly clumsy and a coward. He studies medicine. Sometimes his boasting gets intense. At times, he becomes so greasy and cheesy with his bragging that it's unbearable (though not always). He occasionally shows a protective, guardian-like side toward Ji-oh and Pipi. He has yellow hair tied in a ponytail.

Dr. Brain is a self-proclaimed genius inventor (though maybe not) and a Doctor of Medicine. As Kei's boss, he devises new treatments every day. He enters Pipi's body with Ji-oh inside the Hippocrates machine. He is kind, doesn't use profanity, and is an elderly man. He is a doctor who loves candy. He is male, eccentric, and smart (though he acts like a fool sometimes). People call him 'Doctor'. He is a bit of a fool at times. His physical constitution acts up when he's nervous. He likes eating, but not as much as Pipi. He is a light eater. He gets lectured by Pipi almost every day. He occasionally shares great chemistry with Ji-oh. He and Ji-oh are always troublemakers. He screams whenever he sees something scary or faces a crisis (just like Ji-oh). He is a man who boasts a lot. He is in his 40s to early 50s. He likes sweets. He is a coward. He is surprisingly clumsy and fearful.

An invention made by Dr. Brain. While Ji-oh and Dr. Brain were on board, they were shrunk, causing the ship and its passengers to enter Pipi's body. Before shrinking, it is larger than an average adult male. It appears in the Human Body, Microbe World, Parasite World, and Sewerage chapters. It is a human body exploration ship that shrinks to nanometer size along with its passengers. It is an all-purpose vessel equipped with various surgical tools, medicines, and medical devices. It has a solar charging function. Its appearance is said to be modeled after a bacteriophage... because of this, white blood cells inside Pipi's body mistook the Hippo Ship for bacteria and almost ate it. In the Human Body chapter, it could only shrink in the lab and operated by returning to original size after 10 seconds of UV exposure, but from the Microbe chapter onwards, it was upgraded to allow size adjustment anywhere using a portable light ray. From the Parasite chapter, it was equipped with human body navigation, making it easy to know the current location even inside blood vessels. It is a bacteriophage-shaped craft capable of shrinking to nano-units.

The hookworm appears as a major internal danger threatening the exploration ship 'Hippocrates' carrying the protagonist Ji-oh and Dr. Brain. This is a survival adventure that unfolds as Ji-oh and Dr. Brain enter Pipi's body. The appearance of the hookworm: The fierce attacks and threatening presence of the hookworm living in the small intestine (duodenum, etc.) are depicted in a thrilling manner.